Recent studies on Magnesium (Mg) alloys have shown their potential as a novel class of biodegradable metallic materials for medical applications, particularly as orthopedic and maxillofacial implants. Materials currently used for these implants include non-degradable metals (e.g., titanium alloys) and bioabsorbable polymers. Although titanium alloys are widely used, their major limitations include stress shielding on surrounding bone, necessity of revision surgeries for implant removal, and distortion on post-operative evaluation by magnetic resonance imaging (MRI). Moreover, these permanent metals release harmful wear particulates, causing implant loosening and failure in the end. In contrast, bioabsorbable polymers, while degradable, often lack the mechanical strength needed for load bearing implants. Breakage of polymeric implants has been reported during and after surgeries, which complicates patient recovery and causes liability concerns. Bioabsorbable polymers also produce acidic degradation products that further contribute to implant failure and tissue inflammation.
The main advantages of Mg-based implants lie in the biodegradable and resorbable nature of Mg, where degradation products of Mg can be excreted or used in metabolic processes, and its similar mechanical properties to cortical bone. Magnesium is one of the most abundant cations in intracellular and extracellular fluids in the body and is essential for bone and tooth formation. The level of Mg in the extracellular fluid ranges between 0.7 and 1.05 mmol/L, where homeostasis is maintained by the kidney and intestine. Although Mg is a promising material for load bearing medical implant applications, it degrades much too rapidly in physiological conditions to meet the clinical requirements. The degradation rate of Mg alloys should be slow enough so that the load bearing properties of the implants are not compromised prior to tissue regeneration. Rapid degradation causes premature mechanical failure or detachment of implants and an increase in local pH. Mg-based alloys have been actively explored and reported to decrease the rapid degradation compared to pure Mg, while promoting mechanical properties and retaining osteoconductivity leading to enhanced bone formation. Therefore, Mg-based alloys should be further studied and optimized to improve biodegradation, bioactivity, biocompatibility, and mechanical properties for orthopedic and maxillofacial implants.